Technical Paper 9 Slim-profile double glazing - Historic Scotland

Historic Scotland Technical Paper 9
Conclusions
The key findings from the three research reports in this paper are summarised below.
More detailed conclusions and recommendations are presented at the beginning of
reports 1 and 2, and at the end of report 3.
Please note that, in the summary below where ‘Georgian windows’ and ‘Victorian windows’
are stated, this should be read as ‘Georgian style windows with “6 over 6” glass panes’ and
‘Victorian style windows with “1 over 1” glass panes’.
• The best thermal performance was calculated for the window fitted with vacuum double
glazing. (Refer to report 3.)
• The calculated thermal performances of single‐glazed windows with secondary glazing
were not as good as those of windows with vacuum double glazing, but better than the
other slim‐profile double‐glazing system (with one minor exception). (Refer to report 3.)
• Better thermal performances were calculated for slim‐profile double glazing when fitted
into ‘Victorian windows’ compared to ‘Georgian windows’. (Refer to report 3.)
• The centre‐of‐pane U‐values of the slim‐profile double glazing measured in‐situ ranged
from 1.0 to 2.8 W/m 2 K compared to 5.4 W/m 2 K for single glazing. Most systems
achieved a U‐value close to 2.0 W/m 2 K. (Refer to report 1.)
• For windows retrofitted with slim‐profile double‐glazing systems, the calculated wholewindow
U‐values ranged from 1.9 to 3.4 W/m 2 K for ‘Georgian windows’, and 1.4 to
3.0 W/m 2 K for ‘Victorian windows’. This is equivalent to a calculated reduction in heat
loss of 35 to 63 % for ‘Georgian windows’, and 41% to 73 % for ‘Victorian windows’,
compared to their single‐glazed equivalents. (Refer to report 3.)
• For equivalent single glazed windows retrofitted with secondary glazing, the calculated
whole‐window U‐values were 2.0 and 2.1 W/m 2 K for ‘Georgian windows’ and ‘Victorian
windows’ respectively. This is equivalent to a calculated reduction in heat loss of
61 % for ‘Georgian windows, and 59 % for ‘Victorian windows’, compared to their singleglazed
equivalents. (Refer to report 3.)
• For equivalent single‐glazed windows, whole‐window U‐values of 5.2 and 5.1 W/m 2 K
were calculated for ‘Georgian windows’ and ‘Victorian windows’ respectively. (Refer to
report 3.)
• Inert gases account for a significant proportion of the embodied energy in most double
glazing systems due to the energy‐intense processes needed to extract them from the
air. Xenon in particular carries a very high embodied energy. (Refer to report 2.)
• The frames of new sashes also add to the embodied energy. This makes retrofitting into
existing sashes a more sustainable option (than sash replacements). (Refer to report 2.)
• Further research is required to establish the manufacturing energy of double‐glazed
units with vacuum cavities. (Refer to report 2.)
• Manufactures should adopt a more systematic approach to the design of glazing units,
when filled with gas, in order to optimise thermal performance. (Refer to report 3.)
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